The present invention relates to a mobile radio system which has a plurality of radio channels for transmitting control information and message information between base stations and mobile stations. More specifically, the invention relates to a method in which the responsibility for transmitting message information to a mobile station is handed over from at least a first base station transmitter to at least a second base station transmitter.
It is desirable that a mobile radio system has high traffic handling capacity and a high degree of coverage. The traffic handling capacity of a mobile radio system is dependent, inter alia, upon the number of available radio channels and how effectively these channels may be utilised. It is known to arrange several base stations with small covering areas close to each other in a mobile radio system. Available radio channels can then be utilised in a more efficient way for handling peak traffic within a limited geographical area than if base stations with large covering areas are arranged far away from each other in the mobile radio system. Arranging several base stations close to each other can also increase the accessibility in a mobile radio system, particularly if the covering areas of the base stations are mutually overlapping. Two base station with such overlapping covering areas cannot, however, normally use the same radio channel for communication with different mobile stations.
When a mobil station moves from the area covered by one base station to the area covered by another base station in a mobile radio system, the responsibility for communication with the mobile must be transferred from one base station to another. Such transfer in conjunction with an established connection to the mobile station is usually called xe2x80x9chandoverxe2x80x9d.
Handover can be a critical function in known traditional mobile radio systems. There is a risk of the established connection being temporarily interrupted or completely lost in conjunction with the handover. There is usually required comparatively extensive signalling in the mobile radio system in conjunction with changing of the radio channel at handover. The degree of coverage of a mobile radio system depends, inter alia, on the presence of radio shadows and how the covering areas of the individual base stations overlap each other. The possibility of establishing new connections and maintaining established connections to/from mobiles also depends on the presence of reflections and interference.
For increasing the degree of coverage in a mobile radio system, it is known to transmit substantially the same information to a mobile from two or more base transmitters and to receive information from a mobile at two or more base receivers. The publication xe2x80x9cRF PERFORMANCE OF MACROSCOPIC DIVERSITY IN UNIVERSAL PORTABLE RADIO COMMUNICATIONS: FREQUENCY REUSE CONSIDERATIONSxe2x80x9d by Richard C. Bernhardt, from IEEE International Conference on Communications, Jun. 22-25, 1986, Toronto, Canada describes different types of cells and placing of transmitters and the result of simulations of such a system. The publication xe2x80x9cUSER ACCESS IN PORTABLE RADIO SYSTEMS IN THE NOISE LIMITED ENVIRONMENTxe2x80x9d by Richard C. Bernhardt, from ICC""87, Jun. 1987, Seattle, Wash., U.S.A., describes different types of cell  cells and placing  placement of transmitters as well as comparisons between different criteria and algorithms for the selection of the base transmitter.
U.S. patent application Ser. No. 07/315,561 describes a mobile radio system and a method of transmitting message information where at least two bases  base transmitters at least partially simultaneously transmit substantially the same message information to a mobile. The object with this is, inter alia, to enable the degree of coverage to be made greater without the cells in a mobile radio system needing to be reduced, which gives greater freedom in the selection of cell pattern, and fewer handovers. The intention is that the mobile radio system shall be given better possibilities of establishing new connections and maintaining already established connections. According to the patent application, these objects are achieved by certain relationships between modulation of radio signals and parameters of adaptive equalisers in the mobiles and the distance between base transmitters which transmit the same message information.
In cellular mobile radio systems available channels may be allocated to cells and mobiles in various ways. In systems with fixed channel allocation a base for a particular cell may only use certain predetermined channels. Bases  Base transmitters in adjacent cells use different channels. Bases  Base transmitters in different cells sufficiently distant from each other may use same channels according to a channel re-use or allocation plan. In systems with adaptive channel allocation some or all channels are a common resource to all base stations  station transmitters, which means that some or all channels may be used by any base transmitter as long as the radio transmission conditions permit, i.e. certain carrier to interference or noise level.
At least in certain mobile radio systems, it is a problem to make the handover so that the risk of temporary interruption in a connection or that the connection is lost in conjunction with handover is suffiently small. This problem is particularly great with peak traffic. Another problem in connection with handover is the comparatively large need for signalling in the mobile radio system. The object of the invention is, inter alia, to ameliorate these problems.
One object of the present invention is to achieve a handover where the risk of temporary interruption or lost connections can be sufficiently small.
Another object of the present invention is to achieve a handover where the need of signalling via radio signals in the mobile radio system may be comparatively small.
A still further object of the present invention is to achieve a handover which is particularly suitable for mobile radio systems where two or more base transmitters spaced from each other transmit essentially similar message information to a mobile.
Yet another object of the invention is to achieve a handover which is particularly suitable for mobile radio systems with a high degree of coverage and large overlapping between the covering areas of different base transmitters.
In a method in accordance with the invention the responsibility of transferring message information to a mobile station, is handed over from at least one first base transmitter to at least a second base transmitter. Distinguishing for a method in accordance with the invention, and particularly preferred embodiments thereof will be seen from the independent and the dependent claims. Somewhat simplified, it may be said that in a method in accordance with the invention, the same radio channel is preferably used for transmitting message information to the mobile before and after the handover.
The handover preferably takes place without the mobile being informed beforehand that the handover is going to take place. According to a further preferred embodiment of the invention, the radio transmission of message information from a second base station transmitter to the mobile station is started before transmission of message information to the mobile from a first base station transmitter is terminated. Essentially the same message information is thus transmitted during a transition time to the mobile both from a first and a second base station transmitter.
In mobile radio systems where at least two base station transmitters are simultaneously responsible for transmitting message information to at least certain mobiles, it may be preferred not to start or interrupt the transmission from several base transmitters simultaneously in conjunction with the handover. The transmission of message information to the mobile is then preferably started from one of the second base transmitters before initiating transmission of message information from another of the second base transmitters. In such a case, the transmission of message information to the mobile from one of the first base transmitters is preferably terminated before the transmission of such information is terminated from another of the first base transmitters. Initiation and termination of the transmission from the base transmitters preferably takes place in an order such that at least two base transmitters transmit essentially the same message information to the mobiles the whole time.
In the cases where transmission from a second base transmitter must take place on another radio channel than the transmission from a first base transmitter, it is preferable that the radio channels are selected such that during the transition time the mobile station can receive the transmissions from the different base transmitters essentially without interference from each other.
Designing the handover of responsibility for transmitting message information to a mobile in accordance with the invention means several advantages, which are accentuated in preferred embodiments.
Perhaps the most important advantage is that in the cases where the same radio channel is utilised by the bases before, during and after the handover, the need for handover signaling via radio signals can be made particularly small. If the mobile sends and receives on the same channels before, during and after the handover, it does not really need to know whether the responsibility for communication has been handed over. In an extreme case, there is thus no need of handover signalling at all via radio signals in conjunction with handover, and all handover signalling could take place in the stationary part of the mobile radio system. Another important advantage is that the risk of temporary interruption in an established connection call to a mobile or that the connection is lost can be made small.
Another advantage is great flexibility with regard to distributing tasks between the mobile and base in conjunction with the handover. Preparations for and initiative to the handover can take place to a greater or lesser extent in the mobile. It will then be possible to form and distribute tasks in conjunction with preparation for, initiation and execution of the handover such that the need of signalling with respect to the handover will be comparatively small.
A still further important advantage with the method in accordance with the invention is that it is particularly suitable in a mobile radio system where at least two base station transmitters are simultaneously responsible for transmitting message information to at least certain mobiles. In such a system the increasing degree of coverage can be utilised to the full in handing over the responsibility in accordance with the invention.
Further advantages ought to be understood by one skilled in the art after having studied the description of preferred embodiments.